In rotary drilling of subterranean wells numerous functions and characteristics are expected of a drilling fluid. A drilling fluid should circulate throughout the well and carry cuttings from beneath the bit, transport the cuttings up the annulus, and allow their separation at the surface. At the same time, the drilling fluid is expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the borehole""s uncased sections. The drilling fluid should also form a thin, low permeability filter cake that seals openings in formations penetrated by the bit and act to reduce the unwanted influx of formation fluids from permeable rocks.
Drilling fluids are typically classified according to their base material. In oil base fluids, solid particles are suspended in oil, and water or brine may be emulsified with the oil. The oil is typically the continuous phase. In water base fluids, solid particles are suspended in water or brine, and oil may be emulsified in the water. The water is typically the continuous phase. Pneumatic fluids are a third class of drilling fluids in which a high velocity stream of air or natural gas removes drill cuttings.
Invert emulsion fluids, i.e. emulsions in which the non-oleaginous fluid is the discontinuous phase and the oleaginous fluid is the continuous phase, are employed in drilling processes for the development of oil or gas sources, as well as, in geothermal drilling, water drilling, geoscientific drilling and mine drilling. Specifically, the invert emulsion fluids are conventionally utilized for such purposes as providing stability to the drilled hole, forming a thin filter cake, lubricating the drilling bore and the downhole area and assembly, and penetrating salt beds without sloughing or enlargement of the drilled hole.
Oil-based drilling fluids are generally used in the form of invert emulsion muds. An invert emulsion mud consists of three-phases: an oleaginous phase, a non-oleaginous phase and a finely divided particle phase. Also typically included are emulsifiers and emulsifier systems, weighting agents, fluid loss additives, viscosity regulators and the like, for stabilizing the system as a whole and for establishing the desired performance properties. Full particulars can be found, for example, in the article by P. A. Boyd et al entitled xe2x80x9cNew Base Oil Used in Low-Toxicity Oil Mudsxe2x80x9d in the Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R. B. Bennet entitled xe2x80x9cNew Drilling Fluid Technology-Mineral Oil Mudxe2x80x9d in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein. Also reference is made to the description of invert emulsions found in Composition and Properties of Drilling and Completion Fluids, 5th Edition, H. C. H. Darley, George R. Gray, Gulf Publishing Company, 1988, pp. 328-332, the contents of which are hereby incorporated by reference.
The emulsifying agent component of the invert emulsion drilling fluid serves to lower the interfacial tension of the liquids so that the non-oleaginous liquid may form a stable dispersion of from droplets in the oleaginous liquid. As used herein, emulsifying agent and surfactant are used interchangeably. Typically emulsifying agents are chemical compounds that are polar on one portion of the molecule and non-polar on another area Such characteristics allow the emulsifying agent to be slightly soluble in both the oleaginous phase and the non-oleaginous phase and thus found at the interfacial surfaces between the two. Because of the unique characteristics of such compounds, the biodegradability is limited. In some cases, the surfactant molecules are toxic to the biodegrading organism.
As a result of the above, there remains and exists an unmet need for a surfactant package that can be used to formulate invert emulsion drilling fluids and yet remain biodegradable.
The present invention is generally directed to an invert emulsion drilling fluid that is formulated using a biodegradable surfactant. As disclosed below, such a fluid includes: an oleaginous continuous phase, a non-oleaginous discontinuous phase, a surfactant that is a fatty acid ester of diglycerol or triglyccrol, and a weighting agent. In one such illustrative embodiment, the surfactant is a di-fatty acid ester of diglycerol in which the fatty acid has the formula RCO2H and R is an alkyl or alkenyl having 10 to 20 carbon atoms. Alternatively, the surfactant is a di-fatty acid ester of triglycerol in which the fatty acid has the formula RCO2H and the R is an alkyl or alkenyl having 10 to 20 carbon atoms. More preferably the surfactant is selected from polyglyceryl-2 diisostearate or polyglyceryl-3 diisostearate or mixtures and combinations of these. One of skill in the art should appreciate that the oleaginous fluid may be selected from a wide variety of suitable materials. Examples include: diesel oil, mineral oil, synthetic oil, ester oils, glycerides of fatty acids, aliphatic esters, aliphatic ethers, aliphatic acetals, or other such hydrocarbons and combinations and mixtures of these and similar fluids. In a similar manner, the non-oleaginous phase may be selected from a wide variety of suitable materials. Examples of which include: fresh water, sea water, brine, aqueous solutions containing water soluble organic salts, water soluble alcohols or water soluble glycols or combinations and mixtures of these and similar fluids. The weighting agent component of such an illustrative drilling fluid can be either a water-soluble weighting agent or a water insoluble weighting agent or combinations and mixtures of these two. In one illustrative embodiment, the water insoluble weighting agent is selected from barite, calcite, mullite, gallena, manganese oxides, iron oxides, or combinations and mixtures of these and similar solid materials used to weight drilling fluids. In another illustrative embodiment, the water soluble weighting agent is selected from water soluble salts of zinc, iron, barium, calcium or combinations and mixtures of these in aqueous solutions used to add weight to drilling fluids.
Also encompassed by the present invention are the methods of formulating such fluids and using such fluids in the drilling of subterranean wells. For example in one such embodiment, a drilling fluid as described below is formulated, circulated in a rotary drilling drill string and utilized as the drilling fluid in drilling a well. 1These and other features of the present invention are more fully set forth in the following description of preferred or illustrative embodiments of the invention.